1. Field of the Invention
The present invention relates to a regulator for powering a load with a predetermined voltage, stable in temperature and independent from possible variations of a supply voltage of the regulator. The present invention more specifically applies to a series regulator which can operate with a low supply voltage, for example, on the order of 2.2 volts and relates, more specifically, to a regulator implemented in the form of a circuit integrating bipolar transistors and MOS transistors.
2. Discussion of the Related Art
FIG. 1 shows a diagram of a conventional series regulator.
The regulator generally includes an output stage 1 for supplying, on a terminal S, a temperature-compensated reference voltage Vref. Stage 1 is controlled by a current source 2 for providing a control current Ib, independent from possible variations of a supply voltage Vcc of the circuit. Voltage Vref is used to power a load Q, either directly, or via a resistive dividing bridge (not shown) connected between terminal S and the ground.
Stage 1 includes an assembly formed of two NPN-type bipolar transistors T1, T2 and of a resistor R1 for setting voltage Vref, by using the energy gap of silicon, independently from the temperature. Such an assembly is generally referred to as a "band gap" assembly. Transistor T1 is connected in series with a P-channel MOS transistor MP1 between a terminal E, receiving supply voltage Vcc, and the ground. The emitter of transistor T1 is connected to the ground and the source of transistor MP1 is connected to terminal E. Resistor R1 is connected in series with transistor T2 between terminal S and the ground. The emitter of transistor T2 is connected to the ground and connection node A of resistor R1 and the collector of transistor T2 is connected to the base of transistor T1. Terminal S is connected, via an N-channel MOS transistor MN2, to terminal E. The source of transistor MN2 is connected to terminal S and its gate is connected to the drain of transistor MP1 and to the collector of transistor T1.
The value of voltage Vref corresponds to the base-emitter voltage drop Vbe1 of transistor T1 plus the voltage drop in resistor R1 (Vbe1+I'.R1).
The current I' in resistor R1 is set by current source 2. This source 2 includes a first branch formed by a P-channel MOS transistor MP3, two NPN-type bipolar transistors T5, T3, and a resistor R3, connected in series between terminal E and the ground. A second branch includes a P-channel MOS transistor MP4 and two NPN-type bipolar transistors T6, T4, connected in series between terminal E and the ground. Transistor T4 is connected as a diode and as a current mirror on transistor T3, their bases being connected to the collector of transistor T4. Transistor MP3 is connected as a diode and as a current mirror on transistor MP4, their gates being connected to the drain of transistor MP3. Transistor MP1 is connected as a current mirror on transistor MP3, its gate being connected to the gate of transistor MP3. Transistor T6 is diode-connected and the bases of transistors T5 and T6 are connected to the collector of transistor T6. The function of transistors T5 and T6 is to limit the "Early" effect, as will be seen hereafter.
Transistor T2 of stage 1 is connected as a current mirror with transistor T4 of source 2, its base being connected to the base of transistor T4. Thus, neglecting the base currents, the collector current I of transistor T4 is, as a first approximation, equal to (Vbe4-Vbe3)/R3, where Vbe3 and Vbe4 represent the respective base-emitter voltages of transistors T3 and T4. This current I thus varies, like resistor R3, in a way directly proportional to temperature.
Voltage Vref, equal to Vbe1+I'.R1 (or I.R1), is thus temperature-compensated, since voltage Vbe1 varies in a way inversely proportional to temperature.
The operation of such a regulator is perfectly well known and will only be reviewed briefly. As a first approximation, a variation of supply voltage Vcc is compensated by a proportional variation of the on-state drain-source resistances of the MOS transistors. Indeed, the drain transistor MP4 is at a fixed potential corresponding to the sum of the base-emitter voltages of transistors T4 and T6. Thus, current I is, as a first approximation, maintained at a constant value according to resistance R3.
The approximation (I=[Vbe4-Vbe3]/R3) is only true for a low supply voltage Vcc of about 2.2 to 2.5 volts. Indeed, if voltage Vcc becomes higher, transistors T5 and T6, which introduce a collector-emitter voltage (Vce) in each branch to make the voltages Vce of the bipolar transistors and the drain-source voltages (Vds) of the MOS transistors negligible with respect to their respective "Early" voltages, are no longer sufficient. The "Early" voltages of the transistors depend on the technology used and are generally on the order of one hundred volts. When voltage Vcc becomes high, voltages Vce and Vds modify the collector currents of the bipolar transistors and the drain currents of the MOS transistors. In this case, the biasing currents of MOS transistor MP3, MP4, and MP1 are modified, which results in an increase of current I and, by this way, in an increase of voltage Vref.
As a specific example, assuming that transistors MP1, MP3, and MP4 have identical dimensions and exhibit a gate length of 12 .mu.m for a gate width of 20 .mu.m, voltage Vref set to approximately 1.2 volts increases by approximately 60 millivolts when voltage Vcc transits from approximately 2.5 to approximately 10 volts.
A conventional solution to reduce the variations of voltage Vref consists of increasing the gate lengths of transistors MP1, MP3, and MP4 to increase their respective drain-source resistances and thus reduce the voltages Vce of the bipolar transistors. A disadvantage of such a solution is that it is increases the size of the regulator implemented in an integrated circuit.
Further, it is not desirable to increase the number of assemblies that limit the "Early" effect (T5 and T6) since the base-emitter voltage drops introduced by transistors T5 and T6 in the branches of current source 2 increase the minimum supply voltage of the regulator.